Microporous polyolefin multi layer film and preparing method thereof

ABSTRACT

Disclosed is a microporous polyolefin multilayer film which can be used as a separator for a battery, and a preparing method thereof. The microporous polyolefin multilayer film is a trilayer microporous film which is prepared by a wetting method to have both surface layers containing polyethylene of 95 wt % or more having a melting temperature of 125° C. or more, and an intermediate layer containing polypropylene of 50 to 90 wt % having a melting temperature of 160° C. or more and polyethylene of 10 to 50 wt % having a melting temperature of 125° C. or more, wherein a thickness of the film is 9-50 μm, a puncture strength is 0.15 N/μm or more, a permeability is 1.5×10 −5  Darcy or more, a multiplication of the puncture strength and the permeability is 0.4×10 −5  Darcy·N/μm or more, a contraction rate in a traverse direction at a temperature of 150° C. for 1 hour is 15% or less, and a melt fracture temperature of 160° C.

TECHNICAL FIELD

The present invention relates to a microporous polyolefin multilayer film having excellent heat resistance, and more particularly, to a microporous polyolefin multilayer film which has a low closing temperature property caused by polyethylene, a high melt fracture temperature property caused by polypropylene, and also has excellent strength, quality stability and heat resistance.

BACKGROUND ART

A polyolefin-based microporous film has been widely used as a battery separator, a separator filter, and a membrane for microfiltration, due to its chemical stability and excellent physical properties.

Among methods to manufacture a microporous film from polyolefin, a wet etching method which makes a single phase by kneading polyolefin with diluent at high temperature, phase-separates the polyolefin from the diluent during a cooling process and then forms pores by extracting the diluent portion is capable of forming a thin film and has excellent strength, permeability, uniform pores, and excellent quality stability, such that it has been widely used in a lithium ion secondary battery, etc.

Although the lithium ion secondary battery is an excellent battery having a very high energy density, it has danger of explosion when a short is generated, such that a separator used therein is greatly requested to have quality stability together with a high quality level. In accordance with the recent trend of high-capacity and high-output of the lithium ion secondary battery such as a battery for a hybrid vehicle, etc., there has been a greater demand for a thermal stability of the separator, in addition to the quality stability of the conventional wet product. This is the reason that If the thermal stability of the separator is deteriorated, the danger of explosion by the melt fracture of the separator according to overheat of the battery is increased.

The thermal stability of the battery is determined depending on a high-temperature contraction rate, a closing temperature and a melt fracture temperature of the separator.

If the separator is excessively contacted at a high temperature, a surface of an electrode is exposed. In this case, the possibility of the occurrence of such an electrical short is increased, and thus the thermal stability of the battery is seriously deteriorated.

The closing temperature is a temperature that microporosities of the separator are closed to prevent current from being flowed further, when the inside temperature of the battery is abnormally increased due to the electrical short and the like. The melt fracture temperature is a temperature that the separator is melt-fractured to allow current to be flowed again, when the temperature of the battery is continuously increased over the closing temperature. For the stability of the battery, it is preferable that the closing temperature is low and the melt fracture temperature is high. In particular, the melt fracture temperature, which is a temperature that can continuously block current in a situation that the explosion of the battery may be caused, has the closet relation with the stability of the battery.

The efforts to improve the thermal stability of the separator have been made continuously.

U.S. Pat. No. 6,949,315 discloses a film that improves thermal stability of a separator by kneading inorganic material such as 5-15 wt % titanium oxide to ultra-high molecular weight polyethylene. However, this method may easily cause problems that kneadability is deteriorated according to the adding of the inorganic material, and pin-holes are generated and quality becomes non-uniform according to the deterioration of the kneadability when being stretched, and may cause deterioration in the physical property of the film such as impact strength, etc., due to a lack of compatibility in the interface between the inorganic material and polymer resin, despite the improvement in the thermal stability in accordance with the addition of the inorganic material. The separator using inorganic material cannot but have the disadvantages as described above.

A separator that is prepared by kneading resin having excellent heat resistance instead of inorganic material is disclosed in U.S. Pat. No. 5,641,565. This technique is to mix 30-75 wt % organic liquid-phase compound and 10-50 wt % inorganic material with a resin compound where polyethylene is mixed with 5-45 wt % polypropylene and then to extract the organic liquid-phase compound and the inorganic material, thereby preparing a separation membrane. Although this technique extracts the inorganic material, it still has the problems at the time of kneading as described above and further causes a problem that physical property is deteriorated due to the addition of polypropylene that does not have kneadability with polyethylene as mentioned in the patent itself Also, this method is added with processes of extracting and removing the used inorganic material, thereby having a disadvantage that the process becomes complicated, and this method needs a relatively high amount of polyethylene in order to obtain a sufficient heat resistance effect, wherein the physical property of the separator is more deteriorated.

As a method to prepare a multilayer separator for improving the thermal stability of a separator, U.S. Pat. No. 5,691,077 discloses a method of making a trilayer separator by laminating polypropylene resin having a high melt fracture temperature (having a high melting temperature) on polyethylene having excellent closing property (having a low melting temperature). Although this separator is excellent in view of thermal property, it has disadvantages of the stretching non-uniformity, the generation of pin-holes, and the increase of thickness deviation, etc. during the preparation method of a fabric film by a low temperature drying method, and also it has not only a deterioration problem in productivity due to the addition of the lamination process performed in a separate process but also a delamination problem due to the defect of the lamination, such that this separator has not been widely used. This method has problems that strength, permeability, quality uniformity, and productivity, which are indispensable for a separator of a secondary battery, are deteriorated, despite excellent heat resistance.

Japanese Patent Laid-Open Publication No. 2002-321323 discloses a microporous polyolefin multilayer on which a polyethylene film and a polyethylene/polypropylene compound film are laminated. However, since a polypropylene content of the polyethylene/polypropylene compound film is low, it is difficult to sufficiently increase the melt fracture temperature.

Japanese Patent Laid-Open Publication No. 2007-088815 and International Patent Laid-Open Publication No. WO2004-089627 disclose multilayer separators that have a microporous polyethylene film prepared using a wetting method as a main layer and have a layer mixed with polyethylene and polypropylene prepared using a wetting method as a surface layer. However, owing to its low crystallinity property, when the polypropylene is used as a surface layer in the wetting method extruded with diluent, a lot of polypropylene wax components are remained in the diluent after being extruded and thus there is a problem that a film and a roll surface are polluted by the wax precipitation during post-processings such as stretching/extrusion, etc, thereby deteriorating quality stability. Furthermore, in these technologies, a contraction rate of each film, which is one of the most important factors in the thermal stability, is left out of consideration.

International Patent Laid-Open Publication No. 2006-038532 discloses a multilayer wetting separator containing inorganic particles. However, this separator also has a complicated kneading process due to the kneading of inorganic material. Also, if inorganic material is added into the surface layer, the inorganic material is separated during processes of stretching/extrusion/winding/slitting, etc. so that it may cause pollution due to inorganic powder and scratch of other surface layers, thereby deteriorating quality stability.

The indispensable properties of a separator for a secondary battery are strength, permeability and quality uniformity, and in recent, there is a great demand for thermal stability additionally. However, the conventional techniques as described above could not have accomplished the quality stability, the strength/permeability and a high thermal stability at the same time.

DISCLOSURE Technical Problem

After having repeated broad studies in order to solve the problems of the prior arts as described above, the present inventors found that a trilayer porous film having a separate internal layer formed of polypropylene and polyethylene and other porous films as both surface layers formed of polyethylene and treated by a heat-setting process can be a multilayer separator that simultaneously has property of a porous film containing polypropylene having excellent heat resistance and quality stability property of a microporous polyethylene film.

Therefore, an object of the present invention is to provide a microporous film for a secondary battery that has very excellent strength, permeability, thermal stability, and quality stability at the same time.

Technical Solution

To achieve the above objects, the present invention provides a method of preparing a microporous polyolefin multilayer film that is a trilayer microporous film, including

(a) melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 95 wt % or more having a melting temperature of 150° C. or more;

(b) melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 50-90 wt % having a melting temperature of 160° C. or more and polyethylene of 10-50 wt % having a melting temperature of 125° C. or more;

(c) molding a melt kneaded in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more and the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more to be a trilayer sheet, so that the composite in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more becomes a surface layer and the composite in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more becomes an intermediate layer;

(d) stretching the trilayer sheet to be a film;

(e) extracting the diluent from the film; and

(f) heat-setting the film,

wherein a thickness of the film is 9-50 μm, a puncture strength is 0.15 N/μm or more, a permeability is 1.5×10⁻⁵ Darcy or more, a multiplication of the puncture strength and the permeability is 0.4×10⁻⁵ Darcy·N/μm or more, a contraction rate in a traverse direction at a temperature of 150° C. for 1 hour is 15% or less, and a melt fracture temperature of 160° C.

The basic theory to prepare a microporous polyolefin multilayer film used in the present invention is as follows.

The wetting method to prepare the microporous film from polyethylene uses a process which fabricates and stretches a sheet by mixing and extruding polyethylene and diluent corresponding thereto in order to form a film and then extracts the diluent using an organic solvent. However, the separator prepared by using polyethylene has a limit in heat resistance, since the melting temperature of polyethylene cannot exceed 135° C. To the contrary, although the melting temperature of polypropylene can rise to 160° C. or more, it has a low crystallinity so that it is difficult to manufacture a film having a high permeability using the wetting method. Furthermore, it has a high closing temperature due to the high melting temperature so that stability is deteriorated. (The closing temperature is a temperature that microporosities of the separator are closed to prevent current from being flowed when the inside temperature of the battery is abnormally increased, and thus it is preferable that the closing temperature is low).

One of methods to overcome the problem is to use polyethylene and polypropylene at the same time. However, since polyethylene and polypropylene have not compatibility with each other, the physical property of the final microporous film is deteriorated upon the kneading. In order to minimize the deterioration of the physical property and also maximize the properties of polyethylene and polypropylene, it is preferable to fabricate the multilayer microporous film which has separate layers formed of polyethylene and polypropylene so that the closing temperature is lowered by the polyethylene layer and the melt fracture temperature is increased by the polypropylene.

If polypropylene is used alone, it is difficult to fabricate the film having the high permeability. If polyethylene is mixed with the polypropylene layer, crystallinity of the polypropylene layer is increased and an interface between polyethylene and polypropylene is cracked during the stretching process, thereby increasing the permeability.

Further, due to the low crystallinity property of polypropylene, when the polypropylene is used as a surface layer in the wetting method extruded with diluent, a lot of polypropylene wax components are remained in the diluent after being extruded and thus there is a problem that a film and a roll surface are polluted by the wax precipitation during post-processings such as stretching/extrusion, etc, thereby deteriorating quality stability. This problem can be solved by using the polypropylene layer as the intermediate layer of the trilayer film.

One of reasons why the inside temperature of the battery is abnormally increased is the electrical short in the battery. The electrical short in the battery can be remarkably reduced by increasing the strength of the separator and reducing the high-temperature contraction rate. The strength of the separator is controlled through the stretching process, and the contraction rate is minimized through the heat-setting process.

Consequently, the multilayer microporous film prepared as described above can have a low closing temperature of polyethylene and a high melt fracture temperature of polypropylene at the same time, and also have the excellent physical property and quality stability.

Hereinafter, the respective processes for fabricating the microporous polyolefin multilayer film will be described in more detail.

(a) the melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 95 wt % or more having a melting temperature of 150° C. or more, is performed.

The polyethylene used in the present invention has a melting temperature Tm of 125° C. or more, preferably 125 to 135° C., and may be a polyethylene homo-polymer, a polyethylene co-polymer copolymerized with ethylene and α-olefin comonomer having 3-8 carbon atoms, or a mixture of the homo-polymer and the copolymer or the copolymer and the copolymer. The α-olefin comonomer having 3-8 carbon atoms may include propylene, 1-butene, 1-hexene, 1-butene, and 4-methylpentene-1 and the like. It is preferable that the polyethylene has a weight average molecular weight of two hundred thousand to three million. When the weight average molecular weight is less than two hundred thousand, the physical property of the final porous film becomes weak, and when the weight average molecular weight is more than three million, extrusion kneading is deteriorated so that productivity is decreased. The more preferable range of the weight average molecular weight is two hundred thousand to one million and five hundred thousand.

The diluent used in the present invention may use all of organic liquid phase compounds that form a single phase with resin at an extrusion processing temperature. For example, there are aliphatic such as nonane, decane, decalin, and paraffin oil, etc., or phthalic acid ester such as cyclic hydrocarbon, dibutyl phthalate, and dioctyl phthalate, etc. Preferably, paraffin oil that is harmless to human body and has a high boiling point and less volatile components is suitable, and more preferably, paraffin oil having kinetic viscosity of 20 cSt-200 cSt at a temperature of 40° C. is suitable. If the kinetic viscosity of paraffin oil is above 200 cSt, it leads to a high kinetic viscosity during an extrusion process so that it may cause problems of increase in load and defects on a surface of a film and a sheet, etc., and it leads to difficulties in extraction during an extraction process so that it may cause problems of deterioration in productivity and reduction in permeability due to remaining oil, etc. If the kinetic viscosity of paraffin oil is below 20 cSt, it leads to difficulties in extrusion processing due to viscosity difference with melted polyethylene within an extruder.

The resin compound used in the present invention contains polyethylene of 95 wt % or more having a melting temperature of 125° C. or more, and most preferably polyethylene alone. Other resins like polymethylpentene may be added in the range not to affect the low closing temperature property and the pore structure of polyethylene, and the content of other resins is preferably below 5 wt %. If the content is above 5 wt % (that is, if polyethylene below 95 wt %), the closing temperature property and the pore structure of polyethylene may be not provided, and also the physical property is remarkably reduced.

The preferable composition of the diluent and the resin compound containing polyethylene of 95 wt % or more having a melting temperature of 125° C. or more used in the present invention is constituted by the resin compound of 20-50 wt % and the diluent of 80-50 wt %. If the content of the resin compound is above 50 wt % (that is, if the diluent is below 50 wt %), the porosity is reduced, the pore size becomes small and the interconnection between the pores is few so that the permeability is noticeably deteriorated. To the contrary, if the content of the resin compound is below 20 wt % (that is, if the diluent is above 80 wt %), the kneading of the resin compound and the diluent is deteriorated so that the resin compound is not thermodynamically kneaded in the diluent but is extruded in a gel shape, thereby causing problems of the fracture and the non-uniformity in thickness at the time of stretching.

General additives for improving specific functions, such as oxidation stabilizer, UV stabilizer, and antistatic agents, etc., may be further added into the composition, as needed.

The composition is melted and kneaded using a biaxial compounder, a kneader, or a banbary mixer, etc. that is designed for kneading resin compound and the diluent. The melt kneading temperature is preferably 180° C. to 300° C. The resin compound and the diluent may be inserted into the compound by being previously blended or may be inserted thereto from separated feeders, respectively.

(b) the melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 50-90 wt % having a melting temperature of 160° C. or more and polyethylene of 10-50 wt % having a melting temperature of 125° C. or more, is performed.

The polypropylene used in the present invention has a melting temperature Tm of 160° C. or more, preferably 160 to 180° C., and may be a polypropylene homo-polymer, a polypropylene co-polymer copolymerized with propylene, ethylene and α-olefin having 4-8 carbon atoms, or a mixture of the homo-polymer and the copolymer or the copolymer and the copolymer. It is preferable that the polypropylene has a weight average molecular weight of fifty thousand to three million. When the weight average molecular weight is less than fifty thousand, the strength of the diluent compound is weak and thus it is impossible to perform the stretching process, and when the weight average molecular weight is more than three million, the kneading of diluent and polyethylene is got into trouble.

In the compound of polypropylene and polyethylene, if the content of polypropylene is below 50 wt %, it is not possible to form a polypropylene matrix which is formed into a net of polypropylene, and thus the melt fracture temperature is not remarkably increased. To the contrary, if the content of polypropylene is above 90 wt %, the permeability is considerably reduced due to the low crystallinity property.

In the preferable composition of the diluent and the resin compound of polypropylene and polyethylene, if the content of the resin compound is above 50 wt %, the porosity is reduced, the pore size becomes small and the permeability is noticeably deteriorated. To the contrary, if the content of the resin compound is below 20 wt %, the kneading of the resin compound and the diluent is deteriorated so that the resin compound is not thermodynamically kneaded in the diluent but is extruded in a gel shape, thereby causing problems of the fracture and the non-uniformity in thickness at the time of stretching.

General additives for improving specific functions, such as oxidation stabilizer, UV stabilizer, and antistatic agents, etc., may be further added into the composition, as needed.

The composition is melted and kneaded using a biaxial compounder, a kneader, or a banbary mixer, etc. that is designed for kneading the polypropylene and polyethylene and the diluent. The melt kneading temperature is preferably 180° C. to 300° C. The resin compound and the diluent may be inserted into the compound by being previously blended or may be inserted thereto from separated feeders, respectively.

(c) the molding a melt kneaded in the melting and kneading of the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more and the melting and kneading of the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more to be a trilayer sheet so that the composite in the melting and kneading of the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more becomes a surface layer and the composite in the melting and kneading of the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more becomes an intermediate layer, is performed.

The method to prepare a molding in a sheet shape from the melt may use all of a general casting method or a calendaring method. The temperature of the proper casting or calendering roll is 30° C. to 80° C. If the cooling roll is below 30° C., wrinkles of the sheet may occur due to the rapid cooling of the sheet, and if the cooling roll is above 80° C., surface defects, etc., may occur due to the insufficient cooling.

The method to manufacture the multilayer sheet may use a general co-extrusion method, a thermal bonding method or a coating method. The co-extrusion method is to manufacture a multilayer sheet by co-extruding melts extruded from each extruder through a multi-layered T-die when molding the sheet, and the thermal bonding method is to overlap sheets obtained from each extruder and then thermally bond them, while applying pressure thereto, and the coating method is to fabricate a multilayer sheet by extruding a secondary sheet on a first sheet.

The multilayer sheet is constituted into a trilayer sheet in which the sheet prepared in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more is used as both surface layers, and the sheet in the melting and kneading of the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more is used as an intermediate layer. As described above, if the sheet in the melting and kneading of the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more is used as an intermediate layer, the polypropylene wax components are precipitated so that a film and a roll surface are polluted during the post-processings such as stretching/extrusion, etc.

(d) the stretching the trilayer sheet to be a film is performed.

The stretching may be performed by any stretching methods, such as a tentor type simultaneous stretching or a tentor type successive stretching that performs a primary stretching in a longitudinal direction using a roll and performs a secondary stretching in a traverse direction using a tentor, etc. Stretching ratios are 4 times or more in a longitudinal direction and in a traverse direction, respectively, and a gross stretching ratio is preferably 25-60 times. When the stretching ratio in one direction is below 4 times, orientation in one direction is not sufficient and at the same time, the balance of physical property between the longitudinal direction and the traverse direction is broken so that puncture strength is deteriorated. Also, when the gross stretching ratio is below 25 times, a non-stretching occurs, and when the gross stretching ratio is above 60 times, the possibility that a fracture occurs during the stretching is high and the contraction rate of the final film is increased.

The stretching temperature varies depending on the composite of the melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 95 wt % or more having a melting temperature of 150° C. or more, i.e., a melting point of polyethylene and a concentration and sort of diluent, that are used. The optimal stretching temperature is preferably selected from a temperature range that a crystal portion of 30-80 wt % in polyethylene of the composite and the diluent layer is melted. The extent that the crystal portion is melted depending on the temperature may be obtained from a differential scanning calorimeter (DSC) analysis of a sheet molding. If the stretching temperature is selected from a temperature range which is lower than the temperature that the crystal portion of 30 wt % in polyethylene of the surface layer and the diluent layer is melted, the film does not have softness so that the extension becomes bad, thereby having a high possibility that a fracture occurs at the time of stretching simultaneously with generating a non-stretching. To the contrary, If the stretching temperature is selected from a temperature range higher than the temperature that the crystal portion of 80 wt % of the polyethylene and diluent in the molding of the multilayer sheet surface layer is melted, stretching is easy and non-stretching hardly occurs, but a thickness deviation occurs due to partial over-stretching and the orientation effect of resin is few, thereby noticeably deteriorating the physical property. The stretching temperature range is lower than the temperature that polypropylene is melted, but a low-temperature stretching of polypropylene may be performed. Through this stretching, polypropylene used in the intermediate layer is not fractured but is stretched, and at the same time, the polypropylene matrix is formed at the intermediate layer, thereby increasing the melt fracture temperature and thus improving the thermal stability of the battery.

(e) extracting the diluent from the film is performed.

The sheet of which a thickness becomes thin through the stretching process, that is, the film, is extracted and dried using an organic solvent. In the present invention, the usable solvent is not specifically limited, but any solvent that can extract diluent used in extruding resin, and it is preferable that the solvent is methyl ethyl ketone, methylene chloride, and hexane, etc., that have a high extraction efficiency and a rapid drying. The extraction method may use all general solvent extraction methods such as an immersion method, a solvent spray method, and ultrasonic method, respectively or combinationally. When performing the extracting process, the content of remaining diluent should be 1 wt % or less. If the content of remaining diluent is above 1 wt %, the physical property is deteriorated and the permeability of the film is reduced.

The amount of remaining diluent is greatly influenced depending on an extraction temperature and an extraction time. The extraction temperature is preferably high in order to increase solubility of diluent and solvent, more preferably, 40° C. or less in consideration of a safety for boiling of the solvent. If the extraction temperature is less than a solidifying point of diluent, the extraction efficiency is noticeably deteriorated, and thus the extraction temperature has to be higher than the solidifying point of diluent. The extraction time varies depending on the film thickness, but preferably, 2-5 minutes when manufacturing a microporous film having a thickness of 9-50 μm.

(f) the heat-setting the film is performed.

The dried film is treated by a heat-setting process in order to reduce the contraction rate of a final film by finally removing remaining tension. Generally, the heat-setting process that fixes the film and applies heat thereto is to remove remaining tension by compulsorily holding the film to be contracted, or stretching or contracting the film. In the present invention, the heat-setting process is divided into two processes that firstly stretches the film and secondly contract the film.

A high heat-setting temperature is advantageous for lowering the contraction rate and increasing the puncture strength, but when the heat-setting temperature is excessively high, the film is partially melted and thus micropores that are formed in the film are choked, thereby deteriorating the permeability. The heat-setting temperature is preferably selected from a temperature range that the crystal portion of 10-30 wt % of the film is melted. If the heat-setting temperature is selected from a temperature range lower than the temperature that the crystal portion of 10 wt % of the film is melted, the reorientation of molecules in the film is insufficient so that it does not have a removal effect of remaining tension of the film, and if the heat-setting temperature is selected from a temperature range higher than the temperature that the crystal portion of 70 wt % of the film is melted, the micropores are choked by the partial melting so that the permeability is deteriorated.

The stretching and contracting processes are respectively performed using a tentor type apparatus. Firstly, the stretching is carried out so that the sheet is stretched by 20 to 50% in a traverse direction, thereby increasing the permeability and improving the tensile strength and the puncture strength. If the film is stretched over 50%, the permeability and the strength are improved, but orientation in the traverse direction is increased such that the contraction rate is increased and the pore size is excessively increased. Secondly, the contracting is carried out so that a width of the stretched film is contracted by 10 to 40%. Thus, the tension and the orientation of resin are relieved due to the contraction in the traverse direction. At this time, if the film is contracted above 40%, the permeability and the strength are excessively reduced, and if the film is contracted below 10%, the tension and the orientation of resin are not relieved, and thus since the contraction rate is increased, it is not possible to secure the stability of the battery.

The heat-setting time may be relatively short when the heat-setting temperature is high, and the heat-setting time may be relatively long when the heat-setting temperature is low. The heat-setting time is preferably 15 seconds to 2 minutes, more preferably, 1 to 2 minutes in a temperature range that the crystal portion of 10-30 wt % of the film is melted and 20 seconds to 1 minute in a temperature range that the crystal portion of 30-70 wt % of the film is melted.

The microporous polyolefin multilayer film according to the present invention and the preparation method thereof will be described in more detail.

The trilayer microporous film has both surface layers containing polyethylene of 95 wt % or more, and an intermediate layer containing polypropylene of 50 to 90 wt % and polyethylene of 10 to 50 wt %, wherein a thickness of the film is 9-50 μm, a puncture strength is 0.15 N/μm or more, a permeability is 1.5×10⁻⁵ Darcy or more, a multiplication of the puncture strength and the permeability is 0.4×10⁻⁵ Darcy·N/μm or more, a contraction rate in a traverse direction at a temperature of 150° C. for 1 hour is 15% or less, and a melt fracture temperature of 160° C.

If the thickness of the film is below 9 μm, the entire strength is weak and thus it is not suitable for the separator for the secondary battery, and if the thickness of the film is above 50 μm, the permeability is low and thus it is not suitable for the separator for the secondary battery. It is more preferable that the film has a thickness of 9 to 30 μm.

The puncture strength is preferably 0.15 N/μm or more. If the puncture strength is below 0.15 N/μm, the strength is weak and thus it is not suitable for the separator for the secondary battery. The more preferable puncture strength is 0.2 N/μm to 0.5 N/μm.

The gas permeability of the multilayer separator is 1.5×10⁻⁵ Darcy or more. If the gas permeability is below 1.5×10⁻⁵ Darcy, the permeability is not sufficient and thus it is not suitable for a high-capacity/high-efficiency battery. The gas permeability is more preferably 2.5'10⁻⁵ to 12.0×10⁻⁵ Darcy.

The multiplication of the puncture strength and the permeability of the multilayer separator is 0.4×10⁻⁵ Darcy·N/μm or more. If all of the puncture strength and the permeability are low at the same time, the film is weak and the porosity is high, thereby deteriorating the stability of the film. If the multiplication of the puncture strength and the permeability is below 0.4×10⁻⁵ Darcy·N/μm, it is not suitable for a high-capacity/high-efficiency battery.

The pore of the microporous film manufactured by the present invention has an average size of 0.01 μm to 0.2 μm. If the average size is below 0.01 μm, the function of the film as the separator is remarkably deteriorated, and if the average size is below 0.01 μm, the safety and stability of the entirety of the film are hurt. The preferable average size of the micropore is 0.01 μm to 0.1 μm.

The contraction rate of the multilayer separator, which is performed at a temperature of 120° C. for 1 hour in a traverse direction, is less than 15%. Since the separator is fixed in the battery in a longitudinal direction but not fixed in a traverse direction, the contraction in the traverse direction is very important. Further, the contraction rate at a temperature of 120° C. just before the polyethylene is melted is very important. For the high-temperature stability of the battery, it is preferable that the traverse contraction rate of the separator after leaving at a temperature of 120° C. for 1 hour is less than 15%, more preferably 10%, most preferably 5%.

The melt fracture temperature of the microporous film of the present invention is preferably 160° C. or more. This temperature is determined depending on a melting temperature of the used polypropylene. In general, since the heat-resistance test of the secondary battery is estimated at a temperature of 150° C., it is preferable that the melt fracture temperature is 160° C. or more.

Further, it is preferable that a sum of each thickness of the surface layers in the microporous film is 50% or more of the entire thickness, and a thickness of the intermediate layer is 1 μm or more. If the sum is below 50% of the entire thickness, the multiplication of the puncture strength and the permeability is lowered, and if the thickness of the intermediate layer is below 1 μm, the improvement of the heat resistance is deteriorated.

As described above, the microporous film of the present invention simultaneously has the heat resistance property of a film containing polypropylene having excellent heat resistance and the quality stability property of a microporous polyethylene film, thereby providing the excellent strength, permeability, thermal stability, and quality stability at the same time.

Hereinafter, the present invention will be described in more detail through the embodiments set forth herein but the present invention is not limited thereto.

Advantageous Effects

The microporous polyolefin multilayer film according to the present invention as described above not only has the low closing temperature property based on polyethylene and the high melt fracture temperature property based on polypropylene, but also has the uniform quality uniformity due to the uniform microporous property of the separator prepared using a wetting method. Further, the microporous polyolefin multilayer film has the high productivity, high strength/permeability and the low high-temperature contraction rate, making it possible to have remarkable effects when being used in a high-capacity/high-output secondary battery.

DESCRIPTION OF DRAWINGS

The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

FIG. 1 shows a frame for measuring a melt fracture temperature of a microporous film prepared in one embodiment of the present invention; and

FIG. 2 shows a structure where a microporous film is set to a frame for measuring a melt fracture temperature of a microporous film prepared in one embodiment of the present invention.

DETAILED DESCRIPTION OF MAIN ELEMENTS

1: frame

2: microporous film

3: tape

BEST MODE

The molecular weight and the distribution of molecular weight of polyethylene and polypropylene were measured by a high-temperature Gel Permeation Chromatography (GPC) manufactured by Polymer Laboratory.

The viscosity of a diluent was measured by Cannon Automatic Viscometer Model CAV-4.

The method of fabricating a sheet and a film from a raw material were as follows.

Method of Fabricating a Film

A diluent and a resin compound of a surface layer were kneaded in a biaxial compound having φ=46 mm and extruded to have a uniform phase. The kneading temperature was 180-280° C. The resin compound was inserted into a main hoper and the diluent was inserted into an extruder using a sidefeeder.

A diluent and a resin compound of an intermediate layer were previously kneaded in a biaxial compound having φ=30 mm and uniformly extruded from a biaxial compound having φ=15 mm. The kneading temperature was 180-280° C.

Each composite of the surface layer and the intermediate layer, which was melted and extruded from the extruder, was extruded from a T-letter shaped die for co-extrusion, which can fabricate a trilayer sheet, and then molded by a casting roll at a temperature of 30° C. so as to have a necessary thickness. A desirable layer structure was formed by a feed block disposed between the extruder and the die, and a thickness of the each layer was controlled by adjusting an extrusion rate of each extruder.

In order to analyze a phenomenon that a crystal portion was melted depending on the temperature of the molded sheet, a DSC manufactured by Mettler Toledo was used. The analysis conditions were as follows: sample weight was 5 mg; and scanning rate was 10° C./min. In order to separately analyze a melting phenomenon of each layer, a single layer was used.

A simultaneous stretching process was performed on the sheet in a tentor type laboratory stretching machine, while changing a stretching ratio and stretching temperature, wherein the stretching temperature was determined in a temperature range that the crystal portion of 30-80 wt % of the polyethylene and diluent layer is melted based on the result of DSC.

The extraction of the diluent was performed in an immersion method at a room temperature using methylene chloride, having the extraction time of 5 minutes.

The heat-setting was performed in a successive oven while changing the temperature, stretch rate and contraction rate. The heat-setting was performed for 50 seconds, wherein the stretching was performed for 30 seconds and the contracting was performed for 20 seconds.

The thickness of the respective film layers were measured using a Scanning Electron Microscope (SEM). After cooling the prepared film under liquid nitrogen for 20 seconds and suddenly breaking it, the thickness of the prepared film was measured by observing the cross-section thereof.

For the prepared film, the puncture strength, gas permeability, contraction rate at a temperature of 120° C. and melt fracture temperature of the prepared film was measured and the results thereof were represented by the Table below.

Method of Measuring Physical Property

(1) The puncture strength was measured as a strength when a pin having diameter of 1.0 mm fractures a film at a speed of 120 mm/min.

(2) The gas permeability was measured by a porometer (PMI Model CFP-1500-AEL). The present invention used a Darcy's permeability constant. The Darcy's permeability constant was obtained from equation 1 below and nitrogen was used in the present invention.

C=(8 F T V)//(πD ²(P ²−1))   [Equation 1]

Herein, C=Darcy permeability constant

F=Flow velocity

T=Sample thickness

V=Viscosity of gas (0.185 for N₂)

D=Sample diameter

P=Pressure

In the present invention, an average value of Darcy's permeability consonant in the range of 100-200 psi was used.

(3) In order to estimate the contraction rate at a temperature of 120° C., after a separator is cut out in 15 cm×15 cm and a length mark of 10 cm was made on at least 5 cut-out separators in a traverse direction, each separator was interposed between papers and then left for 60 minutes in an oven that was stabilized at a temperature of 120° C. Then, the contraction rate is estimated by measuring a change of a length of each separator in the traverse direction.

Contraction rate (%)=100×(initial length−length after being left at temperature of 120° C.)/initial length

(4) In order to measure the melt fracture temperature of the film, the film (5 cm×5 cm) as shown in FIG. 2 was set to the frame (outer: 7.5 cm×7.5 cm, inner diameter: 2.5 cm×2.5 cm) as shown in FIG. 1 using a polyimide type and was put in a convention oven that is maintained at a preset temperature for 10 minutes, and thereafter, it was observed whether the film is fractured. A highest temperature that the film is not fractured eve though 10 minutes lapsed was defined as a melt fracture temperature.

EXAMPLE 1

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 30 wt % and 70 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 50 wt % and 50 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent were 30 wt % and 70 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 1,100 μm and was stretched at a temperature of 119° C. six times in a longitudinal direction and six times in a traverse direction, totally, thirty-six times.

The heat-setting process after the extraction was performed at a temperature of 125° C. The sheet was stretched 40% in the traverse direction in the stretching process, and contracted 20% in the contracting process comparing with a final width in the stretching process.

The thickness of the final film was 18 μm. The physical property of the obtained separator was represented by Table 1 below.

EXAMPLE 2

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 25 wt % and 75 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 70 wt % and 30 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent were 30 wt % and 70 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 800 μm.

The prepared sheet was stretched at a temperature of 117° C. five times in a longitudinal direction and six times in a traverse direction, totally, thirty times. The heat-setting process after the extraction was performed at a temperature of 125° C. The sheet was stretched 50% in the traverse direction in the stretching process, and contracted 20% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 16 μm. The physical property of the obtained separator was represented by Table 1 below.

EXAMPLE 3

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 2.7×10⁵ and having a melting temperature of 130° C. due to use of comonomer propylene, and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 30 wt % and 70 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 80 wt % and 20 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent were 50 wt % and 50 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 850 μm.

The prepared sheet was stretched at a temperature of 116° C. six times in a longitudinal direction and six times in a traverse direction, totally, thirty-six times. The heat-setting process after the extraction was performed at a temperature of 122° C. The sheet was stretched 50% in the traverse direction in the stretching process, and contracted 35% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 24 μm. The physical property of the obtained separator was represented by Table 1 below.

EXAMPLE 4

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 50 wt % and 50 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 60 wt % and 40 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent were 40 wt % and 60 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 550 μm.

The prepared sheet was stretched at a temperature of 121° C. six times in a longitudinal direction and six times in a traverse direction, totally, thirty-six times. The heat-setting process after the extraction was performed at a temperature of 125° C. The sheet was stretched 20% in the traverse direction in the stretching process, and contracted 10% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 12 μm. The physical property of the obtained separator was represented by Table 1 below.

EXAMPLE 5

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and polymethylpentene having a melting temperature of 245° C. were used as the resin compound, wherein the contents of the two components were 95 wt % and 5 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used as the diluent. The contents of the resin compound and the diluent were 40 wt % and 60 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 60 wt % and 40 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent were 40 wt % and 60 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 450 μm.

The prepared sheet was stretched at a temperature of 121° C. five times in a longitudinal direction and five times in a traverse direction, totally, twenty-five times. The heat-setting process after the extraction was performed at a temperature of 125° C. The sheet was stretched 30% in the traverse direction in the stretching process, and contracted 20% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 16 μm. The physical property of the obtained separator was represented by Table 1 below.

COMPARATIVE EXAMPLE 1

Polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 30 wt % and 70 wt %, respectively.

The diluent and the resin compound having the composition were kneaded in a biaxial compound having φ=46 mm and extruded to have a uniform phase. The kneading temperature was 180-280° C. The resin compound was inserted into a main hoper and the diluent was inserted into an extruder using a sidefeeder. The melted and extruded composites were extruded from the T-letter shaped die for co-extrusion and then molded by the casting roll at a temperature of 30° C. so as to have a necessary thickness. A thickness of the layer was controlled by adjusting an extrusion rate of each extruder. The sheet prepared as a single layer had a thickness of 1,200 μm.

The prepared sheet was stretched at a temperature of 121° C. six times in a longitudinal direction and six times in a traverse direction, totally, thirty-six times. The heat-setting process after the extraction was performed at a temperature of 125° C. The sheet was stretched 40% in the traverse direction in the stretching process, and contracted 5% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 20 μm. The physical property of the obtained separator was represented by Table 2 below.

COMPARATIVE EXAMPLE 2

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 60 wt % and 40 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. was used, and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the polypropylene and the diluent were 30 wt % and 70 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 800 μm.

The prepared sheet was stretched at a temperature of 121° C. six times in a longitudinal direction and six times in a traverse direction, totally, thirty-six times. The heat-setting process after the extraction was performed at a temperature of 128° C. The sheet was stretched 40% in the traverse direction in the stretching process, and contracted 30% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 19 μm. The physical property of the obtained separator was represented by Table 2 below.

COMPARATIVE EXAMPLE 3

In a surface layer 1, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 30 wt % and 70 wt %, respectively.

In an intermediate layer and a surface layer 2, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 40 wt % and 60 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent in the intermediate layer and the surface layer 2 were 40 wt % and 60 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 800 μm.

The prepared sheet was stretched at a temperature of 122° C. six times in a longitudinal direction and six times in a traverse direction, totally, thirty-six times. The heat-setting process after the extraction was performed at a temperature of 123° C. The sheet was stretched 50% in the traverse direction in the stretching process, and contracted 7% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 15 μm. The physical property of the obtained separator was represented by Table 2 below. A curling phenomenon was seriously generated in the prepared film, and polypropylene wax powder was extracted at a roller disposed at a rear end of the extraction process.

COMPARATIVE EXAMPLE 4

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 30 wt % and 70 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 5.7×10⁵ and having a melting temperature of 163° C. and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 60 wt % and 40 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent in the intermediate layer were 30 wt % and 70 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 500 μm.

The prepared sheet was stretched at a temperature of 121° C. 3.5 times in a longitudinal direction and six times in a traverse direction, totally, twenty-one times. The heat-setting process after the extraction was performed at a temperature of 125° C. The sheet was stretched 15% in the traverse direction in the stretching process, and contracted 10% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 17 μm. The physical property of the obtained separator was represented by Table 2 below.

COMPARATIVE EXAMPLE 5

In a surface layer 1 and a surface layer 2, polyethylene having a weight-average molecular weight of 1.7×10⁵ and having a melting temperature of 124° C. due to use of comonomer butene-1, and paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. were used, wherein the contents of the two components were 30 wt % and 70 wt %, respectively.

In an intermediate layer, polypropylene having a weight-average molecular weight of 3.7×10⁵ and having a melting temperature of 145° C. due to use of comonomer ethylene and polyethylene having a weight-average molecular weight of 3.0×10⁵ and having a melting temperature of 134° C. were used as the resin compound, wherein the contents of the two components were 70 wt % and 30 wt %, respectively. And paraffin oil having a kinetic viscosity of 95 cSt at a temperature of 40° C. was used as the diluent. The contents of the resin compound and the diluent in the intermediate layer were 60 wt % and 40 wt %, respectively.

The sheet was prepared with the composites of the surface layer 1, the surface layer 2 and the intermediate layer by the film fabricating method as described above. The prepared trilayer sheet had a thickness of 600 μm.

The prepared sheet was stretched at a temperature of 121° C. five times in a longitudinal direction and five times in a traverse direction, totally, twenty-five times. The heat-setting process after the extraction was performed at a temperature of 120° C. The sheet was stretched 50% in the traverse direction in the stretching process, and contracted 45% in the contracting process comparing with a final width in the stretching process. The thickness of the final film was 23 μm. The physical property of the obtained separator was represented by Table 2 below.

TABLE 1 Classification Example 1 Example 2 Example 3 Example 4 Example 5 Composition of Surface PE (100) PE (100) PE (100) PE (100) PE/PMP layer 1 (wt %) (95/5) Composition of PP/PE PP/PE PP/PE PP/PE PP/PE Intermediate layer (wt %) (50/50) (70/30) (80/20) (60/40) (95/5) Composition of Surface PE (100) PE (100) PE (100) PE (100) PE/PMP layer 2 (wt %) (95/5) Thickness Surface 8 5 10 4 6 (μm) layer 1 Intermediate 2 6 4 4 4 layer Surface 8 5 10 4 6 layer 2 Ratio of Intermediate 11 40 16 33 25 layer (%) Puncture strength (N/μm) 0.27 0.18 0.26 0.33 0.29 Permeability (10⁻⁵ Darcy) 2.3 2.5 1.7 1.5 1.8 Puncture strength × 0.62 0.45 0.44 0.50 0.52 Permeability (10⁻⁵ Darcy N/μm) Contraction rate for 14 10 4 11 5 1 hour at 120° C. (%) Melt fracture temperature 161 165 168 165 165 (° C.) PE: Polyethylene, PP: Polypropylene, PMP: Polymethylpentene

TABLE 2 Comparative Comparative Comparative Comparative Comparative Classification Example 1 Example 2 Example 3 Example 4 Example 5 Composition of Surface PE (100) PE (100) PE (100) PE (100) PE (100) layer 1 (wt %) Composition of — PP (100) PP/PE PP/PE PP/PE Intermediate layer (wt %) (40/60) (60/40) (70/30) Composition of Surface — PP (100) PE (100) PE (100) layer 2 (wt %) Thickness Surface 20 8 5 8 4 (μm) layer 1 Intermediate — 3 10 0.8 15 layer Surface — 8 8 4 layer 2 Rate of Intermediate — 16 — 5 65 layer (%) Puncture strength (N/μm) 0.25 0.38 0.15 0.13 0.15 Permeability (10⁻⁵ Darcy) 2.7 — 4.0 1.2 0.8 Puncture strength × 0.68 — 0.60 0.16 0.12 Permeability (10⁻⁵ Darcy N/μm) Contraction rate for 18 6 22 19 1 1 hour at 120° C. (%) Melt fracture 135 171 154 153 150 temperature (° C.) PE: Polyethylene, PP: Polypropylene,

Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.

INDUSTRIAL APPLICABILITY

According to the present invention, the microporous polyolefin multilayer film not only has the low closing temperature property based on polyethylene and the high melt fracture temperature property based on polypropylene, but also has the uniform quality uniformity due to the uniform microporous property of the separator prepared using a wetting method. Further, the microporous polyolefin multilayer film has the high productivity, high strength/permeability and the low high-temperature contraction rate, making it possible to have remarkable effects when being used in a high-capacity/high-output secondary battery. 

1. A preparing method of a microporous polyolefin multilayer film, comprising: melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 95 wt % or more having a melting temperature of 150° C. or more; melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 50-90 wt % having a melting temperature of 160° C. or more and polyethylene of 10-50 wt % having a melting temperature of 125° C. or more; molding a melt kneaded in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more and the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more to be a trilayer sheet, so that the composite in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 95 wt % or more having the melting temperature of 150° C. or more becomes a surface layer and the composite in the melting and kneading the composite formed of the diluent of 80-50 wt % and the resin compound containing polypropylene of 50-90 wt % having the melting temperature of 160° C. or more and polyethylene of 10-50 wt % having the melting temperature of 125° C. or more becomes an intermediate layer; stretching the trilayer sheet to be a film; extracting the diluent from the film; and heat-setting the film, wherein a thickness of the film is 9-50 μm, a puncture strength is 0.15 N/μm or more, a permeability is 1.5×10⁻⁵ Darcy or more, a multiplication of the puncture strength and the permeability is 0.4×10⁻⁵ Darcy·N/μm or more, a contraction rate in a traverse direction at a temperature of 150° C. for 1 hour is 15% or less, and a melt fracture temperature of 160° C.
 2. The preparing method according to claim 1, wherein a composite formed of a diluent of 80-50 wt % and polypropylene of 20 to 50 wt % having a melting temperature of 125° C. or more is melted and kneaded in the melting and kneading a composite formed of a diluent of 80-50 wt % and a resin compound containing polypropylene of 95 wt % or more having a melting temperature of 150° C. or more. 3-8. (canceled)
 9. The preparing method according to claim 1, wherein a sum of each thickness of the surface layers is 50% or more of an total thickness, and a thickness of the inner layer is 1 μm or more.
 10. The preparing method according to claim 2, wherein a sum of each thickness of the surface layers is 50% or more of an total thickness, and a thickness of the inner layer is 1 μm or more.
 11. The preparing method according to claim 1, wherein the thickness of the film is 9-30 μm the puncture strength is 0.2 N/μm or more, the permeability is 2.5×10⁻⁵ to 12.0×10⁻⁵ Darcy, and the shrinkage in a transverse direction at 120° C. for 1 hour is 10% or less.
 12. The preparing method according to claim 2, wherein the thickness of the film is 9-30 μm the puncture strength is 0.2 N/μm or more, the permeability is 2.5×10⁻⁵ to 12.0×10⁻⁵ Darcy, and the shrinkage in a transverse direction at 120° C. for 1 hour is 10% or less.
 13. The preparing method according to claim 9, wherein the thickness of the film is 9-30 μm the puncture strength is 0.2 N/μm or more, the permeability is 2.5×10⁻⁵ to 12.0×10⁻⁵ Darcy, and the shrinkage in a transverse direction at 120° C. for 1 hour is 10% or less.
 14. A microporous polyolefin multilayer film prepared by the method according to claims 1, wherein a thickness of the film is 9-50 μm a puncture strength is 0.15 N/μm or more, a permeability is 1.5×10⁻⁵ Darcy or more, a multiplication of the puncture strength and the permeability is 0.4×10⁻⁵ Darcy·N/μm or more, a shrinkage in a transverse direction at 120° C. for 1 hour is 15% or less, and a melt fracture temperature of 160° C. or higher.
 15. A microporous polyolefin multilayer film prepared by the method according to claims
 11. 16. A microporous polyolefin multilayer film prepared by the method according to claims
 12. 17. A microporous polyolefin multilayer film prepared by the method according to claims
 13. 